Answer:
C. Postulate.
Explanation:
We are asked to find the term that defines the given statement.
Theorem: We know that a theorem is proved by a chain of reasoning. Ir is not self evident.
Definition: We know that definition is a statement of the exact meaning of a word.
Postulate: We know that a postulate is a statement that is taken to be true without any proofs, therefore, postulate is the correct choice.